1. Field of the Invention
The present invention relates to a spin-valve type magnetoresistive thin film element having an electrical resistance which changes by the relation between the magnetization direction of a pinned magnetic layer and the magnetization direction of a free magnetic layer affected by an external magnetic field. In particular, the present invention relates to a spin-valve type magnetoresistive thin film element in which the saturated magnetoresistive constant of a free magnetic layer is properly controlled to reduce Barkhausen noise.
2. Description of the Related Art
A spin-valve type magnetoresistive thin film element is one type of giant magnetoresistive (GMR) element using giant magnetoresistance, and is used to detect recorded magneticfields from recording media, such as hard disks.
A spin-valve type magnetoresistive thin film element having the simplest configuration includes four layers, that is, a free magnetic layer, a nonmagnetic electrically conductive layer, a pinned magnetic layer, and an antiferromagnetic layer, in that order from the bottom. A hard magnetic bias layer and an electrical lead layer are formed on the two outer faces of the layered configuration.
The antiferromagnetic layer is typically composed of an iron-manganese (FeMn) alloy or a nickel-manganese (NiMn) alloy, and the pinned magnetic layer and the free magnetic layer are typically composed of a nickel-iron (NiFe) alloy. The nonmagnetic electrically conductive layer is typically composed of copper, and the hard magnetic bias layer is typically composed of a cobalt-platinum (CoPt) alloy.
The pinned magnetic layer is formed so as to come into contact with the antiferromagnetic layer. When the antiferromagnetic layer is composed of an FeMn alloy, the formation of the layer is performed in a magnetic field. When the antiferromagnetic layer is composed of a NiMn alloy, the layer is annealed in a magnetic field. As a result, the magnetization of the pinned magnetic layer is pinned in the height direction (the direction of the leakage or fringing magnetic field from the recording medium) to generate a single-domain state. The magnetization of the free magnetic layer is oriented in the track width direction by a biasing magnetic field from the hard magnetic bias layer. The relative angle defined by the magnetization of the free magnetic layer and the magnetization of the pinned magnetic layer is 90.degree..
In the spin-valve type magnetoresistive thin film element, a sensing current is applied from the electrical lead layer to the pinned magnetic layer, the nonmagnetic electrically conductive layer and the free magnetic layer. When a fringing magnetic field is applied from the recording medium, the magnetization of the free magnetic layer varies from the track width direction to the direction of the fringing magnetic field. A change in the magnetization direction in the free magnetic layer causes a change in electrical resistance of the element, in connection with the magnetization direction of the pinned magnetic layer.
In the spin-valve type magnetoresistive thin film element composed of metallic layers, the upper and lower faces and a height side face are covered with an insulating or gap layer composed of, for example, Al.sub.2 O.sub.3, and another side face at the air-bearing surface (ABS) side or at the front side, which is away from the height side face, is exposed. Thus, a tensile stress in the height direction is applied to the central region of the free magnetic layer of the spin-valve type magnetoresistive thin film element, whereas a compressive stress is applied to two end regions of the free magnetic layer.
As described above, a hard magnetic bias layer, magnetized in the track width direction, is formed on two sides of the free magnetic layer, and the hard magnetic bias layer unifies the magnetization of the free magnetic layer in the track width direction.
The effect of the hard magnetic bias layer is most noticeable at two end regions of the free magnetic layer, and is moderated towards the central portion of the free magnetic layer, away from the hard magnetic bias layer. Thus, the central portion of the free magnetic layer has a large magnetoelastic effect defined by the stress and the magnetostriction constant applied to the free magnetic layer.
The magnetostriction constant of the free magnetic layer has a positive value due to the tensile stress in the height direction applied to the central region of the free magnetic layer. As the positive magnetostriction constant increases, anisotropic magnetic dispersion in the height direction due to the magnetoelastic effect increases, and thus the height direction becomes the easy axis of the magnetization. Such a state facilitates inclination of the magnetization in the height direction in the central region of the free magnetic layer and thus facilitates generation of Barkhausen noise.
On the other hand, a negative magnetostriction constant of the free magnetic layer desirably causes an easy axis of magnetization to be in the track width direction in the central region of the free magnetic layer. Anisotropic magnetic dispersion due to the magnetoelastic effect, however, occurs at the two end regions of the free magnetic layer in which a compressive stress is applied in the height direction, and hence, the height direction will be an easy axis of magnetization.
Since the two end regions of the free magnetic layer are significantly affected by the hard magnetic bias layer, which is magnetized in the track width direction, the anisotropic magnetic dispersion in the height direction due to the magnetoelastic effect is considered to be slight compared to that in the central region of the free magnetic layer. When the free magnetic layer has a large positive magnetostriction constant, the anisotropic magnetic dispersion becomes considerably large in the height direction due to the magnetoelastic effect.
In such a state, magnetization at the two end regions of the free magnetic layer tends to incline in the height direction and facilitates the generation of Barkhausen noise.